A person without the ability to pay court costs has the option to swear a pauper's oath in order to file a lawsuit without paying filing fees. Prisoners filing legal actions often use a pauper's oath because persons in prison are often completely without money or any means of acquiring any.
One pauper's oath used when establishing indigent status under United States Federal law is as follows:
I do solemnly swear that I have not any property, real or personal, exceeding $20, except such as is by law exempt from being taken on civil process for debt; and that I have no property in any way conveyed or concealed, or in any way disposed of, for my future use or benefit. So help me God.